The basic theory and operation of thermoelectric devices have been developed for many years. Thermoelectric devices are essentially small heat pumps which follow the laws of thermodynamics in the same manner as mechanical heat pumps, vapor compressors associated with conventional refrigerators, or other apparatus used to transfer energy. A thermoelectric device can function as a cooler, heater, power generator or thermal energy sensor.
Modern thermoelectric devices typically have solid state electrical components as compared to more traditional mechanical/fluid heating and cooling components. The circuit for a modern thermoelectric device generally includes two dissimilar materials such as N-type and P-type thermoelectric semiconductor elements typically arranged in an alternating N-element and P-element configuration. The thermoelectric elements are generally coupled electrically in series and thermally in parallel. The Peltier effect occurs when voltage is applied to the N-type and the P-type elements resulting in current flow through the serial electrical coupling. The serial current flow results in heat transfer across the N-type and P-type elements in the parallel thermal coupling. The direction of current flow through the thermoelectric elements determines the direction of heat transfer by the thermoelectric elements.
The efficiency of a thermoelectric device typically decreases with an increase in the difference in temperature across the associated thermoelectric elements. To achieve a minimal temperature difference across the thermoelectric elements, a first heat-sink (sometimes referred to as the "hot sink") is preferably coupled to the hot side of the thermoelectric device to aid in dissipating heat from the thermoelectric elements to the adjacent environment. In a similar manner, a second heat sink (sometimes referred to as a "cold sink") is often coupled to the cold side of the thermoelectric device to aid in removing heat from the adjacent environment. Increased cooling of the first heat sink or hot sink further increases the efficiency of the associated thermoelectric device. Natural convection, forced convection, liquid cooling or a combination thereof are currently practiced methods for cooling the hot sink.
For some applications natural convection cooling of a heat sink may not dissipate sufficient heat energy to allow the associated thermoelectric device to operate efficiently. Forced convection cooling enhances natural convection cooling by forcing air flow across the heat transfer surfaces of the heat sink thereby increasing the heat dissipating capability of the heat sink. Unfortunately, forced convection cooling systems require additional hardware and circuitry to control the fans, motors and power convertors along with fittings necessary for forced convection cooling. These additional components add to the complexity of the associated thermoelectric heat transfer system.
Liquid cooling of a heat sink to increase energy dissipation from the heat sink is also currently practiced and involves providing liquid coolant flow through or around the associated heat sink. Unfortunately, such liquid cooling also requires additional hardware and fittings to provide the desired liquid coolant flow.
Boiling a liquid on a surface of a heat sink is another technique to increase energy dissipation from the heat sink. From the perspective of enhanced thermodynamic efficiency associated with boiling a liquid such as a refrigerant, it is often desirable to have vaporization of the liquid take place with very little, if any, superheating of the bulk liquid. Open cell porous coatings have previously been used on some heat exchanger elements to provide an enhanced heat transfer surface which will thermodynamically affect boiling of the liquid. The porous coating on the enhanced heat transfer surface provides a multitude of interconnected open cells which are partially filled with liquid and act as nucleation sites for the growth of vapor bubbles within the boiling liquid. U.S. Pat. No. 3,990,862 entitled "Liquid Heat Exchanger Interface and Method" provides examples of such enhanced heat transfer surfaces.
Various enhanced heat transfer surfaces have previously been used to improve the thermodynamic efficiency with respect to condensing vapors on the associated surfaces. Enhanced heat transfer surfaces have also been used to improve the thermodynamic efficiency of both natural and forced convection cooling of the associated surfaces.
A wide variety of containers and enclosed structures are designed to be maintained within a selected temperature range. Examples of such containers and enclosed structures include, but are not limited to, refrigerators, picnic coolers, cabinets containing sensitive electronic equipment, and organ transplant containers. The use of thermoelectric devices which operate on a DC voltage system is well known to maintain desired operating temperatures in refrigerators, portable coolers, and other types of enclosed structures. An example of a container having a thermoelectric cooler is shown in U.S. Pat. No. 4,726,193 entitled Temperature Controlled Picnic Box. Examples of refrigerators which function with a thermoelectric device are shown in U.S. Pat. No. 2,837,899 entitled Thermoelectric Refrigerator; U.S. Pat. No. 3,177,670 entitled Thermoelectric Refrigerator; and U.S. Pat. No. 3,280,573 entitled Refrigerator--Package Arrangement. U.S. Pat. No. 5,168,339, entitled Thermoelectric Semiconductor Having a Porous Structure Deaerated in a Vacuum and Thermoelectric Panel Using P-Type and N-Type Thermoelectric Semiconductors discloses an electronic refrigerator panel. A more recent example of a thermoelectric refrigerator is shown in U.S. Pat. No. 5,522,216 entitled Thermoelectric Refrigerator.
Conventional refrigerators typically consist of an insulated enclosure with a cooling system based on the vapor compression cycle of fluorinated hydrocarbons, chlorofluorohydrocarbons, or other types of hydrocarbons. The cooling system associated with conventional refrigerators usually has greater cooling capacity than the actual heat load which results in the cooling system acting intermittently in a binary duty cycle--either on or off. This binary duty cycle results in temperature variations as the refrigerator warms up while the compressor is off and cools down when the compressor is running. Thus the temperature in a typical refrigerator is not steady, but cycles between an upper limit and a lower limit. This compressor cycling may reduce the operating efficiency of the associated cooling system.
The negative effects of refrigerants associated with conventional refrigerators on the environment are well known. Both national and international regulations exist to ban the use of some refrigerants such as CFCs. Other fluorocarbons such as HCFCs and HFCs have their own limitations and problems for use in refrigeration systems.